Variable frequency oscillator



FRANK A. HESTER INVEN TOR.

ATTORNEY 2 Sheets-Sheet l R O T A L L M R s P :APIPII'I E O E E m w E N P 2 R 3 H m 5 \6 l Q. A m w F L F E P PR 3 L E Rollo I I B +0 m F M l m Amw mm R mm Q X A A O SS 3 P V V m I a l 0 ll. .0 0E \Mm o m Saw L M May 12, 1953 Flled Aug 10, 1946 May 12, 1953 F. A. HESTER 2,633,550

VARIABLE FREQUENCY OSCILLATOR Filed Aug. 10, 1946 2 Sheets-Sheet 2 SIGNA SOURCE o 7 SIGNAL SOUCE FRANK A. HESTER INVENTOR.

BY M

ATTORNEY Patented May 12, 1953 I V "QvAamBLE FREQUENCY osciLLA'ron I Y Frank Hester, New York, N."Y.,"assignor to Faximile,=-Inc., NewYork, N. ,Y., a corporation of D ela'v'vare Application August 10, 1946; Serial No. 689,792

lation of which may be varied in. response; to. av

Signal source. I

Itis one of the. objects of the invention to vide a vacuum tube-oscillation generator, the.

frequency of which will vary in 'accordancewith the variation of a source of signals;

Another object of the present invention isto provide an oscillator embodying aperiodic circuits which maybe frequency modulated or readily frequency controlled by means of a control signal.

A further object of the invention is to provide a simple and efficient capacitance-resistance oscillator, the frequency of which will vary in accordance with certain changes of a source of signals impressed thereupon.

Another object is to provide a vacuum tube oscillation generator that is simple in construction and yet extremely efiicient for use at very low frequencies.

A further object of the invention is to provide a simple yet eflicient variable-frequency oscillator operable at low frequencies so that a synchronizthese is in the field of facsimile communications, v

In certain types of facsimile communications, the

facsimile signals obtained from a photo-electriccell, when'a facsimile copy is scanned,pmay be applied in such a way as to control the frequency of oscillation of a sub-carrier oscillator in accordance with the facsimile signals. This maybe done, for example, by causing the oscillator frequency to vary in such away that'when the original copy is black, the oscillator frequencyis some low predeterminedvalue and when the copy is white, the frequency of the oscillator is some high predetermined value. When the shades of gray vary between black and white, the] frequency of oscillation will be between the two'piedetermined limits at a frequency determined'by the optical density of the material being scanned.

In this type of system, use of the present-invention will provide a simpler, more eflicient solution to the problem.

"in another application in'the field of facsimile,

it is necessary to provide synchronization be-- tween-the scanning spot at the transmitting end and the recording spot at the receiving end. In one method of doing this, the receiving scanning rate is-controlled by the frequency of a local oscillator. The frequency'of the'local oscillator is controlled by a source of signals obtained from .a synchronizing pulse transmitted at the transmitting end in accordance withthe rate of scanning, and received at the receiving end in such a way as to provide a source of signals for ad-- justing the frequency of the local oscillator to such a value that the scanning at the transmitting end and the receiving end may be in synchronism. The use of the present invention in this'type' of synchronizing system provides a simple, reliable, inexpensive answer to the problem.

In describing the present invention, reference is madeto the drawing accompanying and form ing a part of this specification, and in which:

Fig.1 is a circuit'diagram of a preferred embodiment of this inventiom- I Fig. 2 is a simplified block diagram of the preferred embodiment of the invention which will be useful. in clarifying the explanation of the principles of "operation of the invention.

Fig. 3 is a vector diagram describingthe relationship between certain currents and voltages at various points in the circuit of the presentthe present invention, which will be useful in describing its principles of operation.

Fig. 5'is a plot of the frequency of oscillation of the oscillator against variations" in gain of a typical oscillator.

a' typical oscillator.

Fig. 7 is a circuit'diagram of another embodiment of the present invention.

Fig. 8 is a simplified equivalent circuit, somewhat similar to Fig. 4, but is directed to the modularly to Fig; 1, the two tubes I0 and may be standard'vacuum tubesyforexample, l 8 may be a 68K? and I2 may be a 6J5. type tube. The output I0 is connected to the input of l2 by any standard coupling means such as the B+ blocking- 1 capacitor! l.. and the grid. resistor: 19 shown in the diagram. This coupling means provides substantially no phase shift between the out put of amplifier tube It) and the input of vacuum tube I2. The output of I2 is connected into a plate load resistor l1, a plate load or tank capacitor. I8. and, a. phase shift, feedback network IS -44: providing a substantial phase: shift less than 90 degrees as will be described in connection with Fig. 3. across the resistor I4 is applied to the input of tube thereby completing an oscillating circuit: Plate voltage for tubes l0 and i2 is supplied by voltage source It.

The voltage developed.

The voltage amplification of thetube Ill may accordance with the gain of tube Ill which is varied in accordance with the signal source IS in a manner to be described hereinafter.

Referring to Fig. 2, which is a simplified block diagram of Fig. 1 for clarity, consider the instantaneous values ofthe voltages and currents, E

Ed, E1, and Ip at-the points marked on the diagram.

Referring to Fig. 3, in conjunction with Fig. 2, let the instantaneous value of Es be represented by the vector E? on the diagram. The instantaneous value of voltage E1 will be determined from E? by the-constants l3 and M, which are represented in the diagram (Fig. 3) by the vector E1. This may be shifted in phase from the vector'E throughan angle G'determined by the reactance of 13 relative to the resistance of 1.4.

It Willalsobe reduced in amplitude by an amount determined by the samefactors.

Let the instantaneous voltage amplification of the variable gain amplifier'be represented by the factor K; then the output Es of thevariable-gain amplifier will be shifted in phase 180 from E1 but at an amplitude K times asgreat as E1.

This is represented on the diagram of Fig; 3 as the vector Es.

A component of the plate current In of the.

tube [2 will be in phase'with its input grid voltage EG and determined in magnitude-by themagnitude of Ed. vector in phase with Ed and with an amplitude determined by the amplitude of Ed. This isirepresented on the diagramyof. Fig. 3 by the vector I vector. I may'be resolvedinto two components; one of which IP ,,lS in quadrature-with the vec=- tor E and the other IPR, in phase opposition to the vector E Thus, when a voltage EP is impressed upon the plate, of tube 12, theresulting plate current may be represented as the sum of two components, one in quadraturewith E 'and the other in phase opposition: with. EP. magnitudes of the resulting currents IP and IP are determined in partby the voltage amphfication of the variable'gain amplifier, their magnitudes varying directly with this factor.-

Fig. 4- is a further simplification: for the purpose of clarity, which is based upon. the phase analysis of Fig. 3. The-plate current component IPX maybe represented as that component of current that wouldbe drawn byanwinductance.

This may be represented by a- As is shown in the diagram. of "Fig; 3; the" The 29 having a voltage Ep impressed across it, the magnitude of IPX varying in accordance with variations in the voltage amplification K of the variable gain amplifier.

The value of inductance 29 in the diagram of Fig. 4 is determined by the magnitude of the current. b for a given magnitude of E Hence, the value of inductance 29'varies with the voltage amplification K of the variable gain amplifier. Similarly, the component of plate current I may-be represented in Fig. 4 by the current that would be drawn by a negative resistance 3! whenavoltage-EP i's'applied across its terminals. The magnitude of" 11 is determined by the voltage amplification K' of the variable gain amplifien; hence the value of negative resistance 3! also is, determined by K.

The voltage Ep is also. applied in Fig. 4 as in Fig. 2 and in Fig. 1, to-the resistor ll, the capacitor I8, and to the series combination l3-l4 as well as the'plate resistance 32 of tube 12. Fig. 4 then represents an equivalent circuit of the oscillator-wherein the effect of the tubes [0 and I2 has been replaced by the equivalent effect of the inductance'ZB and a negative resistance 31 connected across the circuit, the value of the inductance 29 and the value of the negative resistance being determined in part by the value of the voltage amplification K' of the variable gain amplifier.

The equivalent circuit of Fig. 4 will have a natural frequency of oscillation determined by the values of its components. The circuit will produce sustained oscillations if the value of 3i is within a proper range of values, the proper range of values being determined in any particular instance by the losses introduced into the circuit by the positive resistors l1, l4, and 32. When negative resistor 3| is of such a value that it willexactly compensate or overcompensate the aforementioned losses,v the circuit will produce sustained oscillations of a frequency determined by the components, including the value of the equivalent inductance 29 which is in turn determined-in part bythe variable amplification factor-K of the variable gain amplifier. Since the voltage amplification factor K is in turn determined by the instantaneous value of the signal source, the equivalent inductance 2B in Fig. 4 is likewise determined by the instantaneous value of the signal source. Hence, the frequency of oscillation of, the circuit is determined by the instantaneous value of the signal source. Figs. 5 and 6 illustrate theserelationships.

InFig. 5, a plot of frequency indicated bythe symbol fagainst the variable amplification factor K of the variable gain amplifier is depicted. Forvalues of K suificiently low, the circuit will not produce sustained "oscillation. This may be visualized in reference to Fig. 4 by the fact that for sufficiently low values of K the value of the negative resistance 3! will not be proper for the maintenance of sustained oscillations. For sufficiently high values, the circuit will produce sus tained oscillations varying with K in accordance with the curve shown in Fig. 5 for a typical oscillator.

Fig. 6 shows a plot of the negative grid voltage E0 in relation to the frequency. It will be seen that as the negative gridvoltage increases negatively,- the frequency will increase. It will also be-seen-that the frequency may be controlled asalinearfunction of the grid voltages EC over a considerable range asshown by the linear portion of the curve.

The circuit shown in Fig. '7 is another embodiment of the present invention differing essentially from the circuit of Fig. 1 in that tank inductance 27 in series with resistance 33 is used to load tube It and the feed-back circuit consists of inductance 24 in series with resistance M. With the proper choice of these components a result equivalent to the operation of the circuit of Fig. 1 may be obtained.

Fig. 8 shows an analysis of Fig. 7, which analysis results in an equivalent circuit similar to that of Fig. 4 but with the capacitor [3 (Fig. 4) replaced by the inductance 24 (Fig. 8), the resistor-capacitor combination H and 18, respectively, (Fig. 4) replaced by the series combination 33 and 2'! (Fig. 8), and the equivalent inductance 29 (Fig. 4) replaced by an equivalent capacitance at (Fig. 8). Again, as in the circuit of Fig. 4, the frequency of oscillation of the circuit shown in Fig. '7 is dependent upon the voltage amplification of the tube It, which is in turn dependent upon the instantaneous value of the signal from the signal source it.

The circuit of Fig. 9 is a third embodiment of the present invention where the phase shift is produced by network 28 and 40 placed between the output of the tube l0 and the input of the tube 12 in conjunction with load I! and iii. The plate current IP of the tube i2 flows through the cathode resistor 39, thereby producing a voltage drop Es which is in phase with the plate current 11 This voltage drop Es is applied to the input of tube ll), thereby completing an oscillating circuit. An analysis similar to that of the analysis given for the circuit in Fig. 1 may be applied to the circuit of Fig. 7 with similar results.

What is claimed as new and desired to be secured by Letters Patent of the United States, is:

A variable frequency oscillation generator comprising a vacuum tube circuit having an input and an output, a reactive tank element connected across the output of said vacuum tube circuit, a variable gain amplifier having a lift-degree phase shift between its input and output, the amplifier being an essential integral component of the oscillation generator, circuit means coupling the output of the vacuum tube circuit to the input of the variable gain amplifier, circuit means coupling the output of the variable gain amplifier to the input of the vacuum tube circuit, one of said circuit means providing substantially no phase shift and the other providing a substantial phase shift less than degrees, and means for varying the gain of said variable gain amplifier, whereby the frequency of the oscillation generator varies as a function of the gain of said integral amplifier.

FRANK A. HESTER.

References Cited in the file of this patent UNITED STATES PATENTS Number 

